The Nexus Biostone Carbon Filter is the best quality, 13-stage, single-body filter that you can buy for Nexus Water Ionizers to use with city water. This is the filter that comes installed in most Nexus water ionizer machines. It can trap a wide variety of contaminants while allowing beneficial minerals to pass through. The Nexus Biostone Carbon Filter uses the highest quality NSF-certified filter media available. It is designed to filter out heavy metals and the most common impurities in city water. The "Biostone" (tourmaline) is designed to reduce the surface tension of the water, naturally, prior to ionization in order to stimulate the best possible ionization.